Lithium ion secondary batteries, in which lithium ions migrate between positive and negative electrodes for charging and discharging, have been attracting increasing importance because they can provide high energy density with light weight, so that they can be preferably used for power supplies for personal computers and mobile terminals, particularly for in-car power supplies.
Negative electrodes for this type of lithium ion secondary battery typically contain, as a material (negative electrode active material) which can reversibly store and release lithium ions, particle carbon materials (carbon particles) having at least partially a graphite structure (layer structure). Typical structures of negative electrodes include, for example, a layer (negative electrode mixture layer) mainly containing carbon particles as a negative electrode active material retained on a negative electrode current collector.
Upon charging lithium ion secondary batteries, lithium ions are generally consumed at the surface of a negative electrode active material for reductive degradation reaction of an electrolyte solution to form a solid electrolyte interface (SEI). The SEI suppresses further reductive degradation reaction of the electrolyte solution at the surface of the negative electrode active material, so that lithium ions can be easily and smoothly enter and leave the negative electrode active material.
However, upon an initial charge, a SEI may be formed as a result of excessive degradation of an electrolyte solution at the surface of a negative electrode active material. In such a case, lithium ions are excessively consumed and the initial efficiency (i.e. battery capacity) of lithium ion secondary batteries may be reduced. In order to address this problem, Patent Literature 1 may be mentioned as prior art. Patent Literature 1 discloses a technique for improving the battery capacity by coating the surface of a negative electrode active material with a phosphate compound polymer in order to suppress the reaction between lithium ions and an electrolyte solution. Patent Literature 2 to Patent Literature 4 may also be mentioned as prior art relating to negative electrode active materials and negative electrodes for lithium ion secondary batteries.